In order to form airgaps by an etch back technique, a top surface of a copper (Cu) structure in a SiCOH (silicon, carbon, oxygen and hydrogen) based interlayer dielectric (ILD) is exposed to an oxidizing plasma which is used to damage or alter the surrounding SiCOH. During this process the Cu surface as well as some part of the underlying microstructure of the Cu which is exposed to the plasma can oxidize as an unwanted side effect. Additionally, such an airgap formation process also has the additional need that nanocolumns formed in the processing of the di-block or cap mask not only need to be transferred through the cap material, but also into and through the ILD material followed by the aforementioned oxidizing plasma to damage the material and fabricate an airgap.
One of the problems with transferring these nanocolumns into the cap material and the underlying ILD material is that the transfer mask may not survive the imaging process long enough to ensure a complete etch or patterning of the nanocolumn into the ILD. Further, the etch process itself could become self limiting due to the steep aspect ratios of the nanocolumns penetrating into the ILD material.
Additionally, the mechanical strength of ILDs may decrease with a decreasing dielectric constant (K). This makes chips with a lower K prone to mechanical failure and requires implementation of expensive processes such as laser dicing to prevent such damage. For ILDs that require e-beam and/or UV beam cure, blanket exposures are currently performed with the main parameter of interest being the dielectric constant of the material. As such, the dielectric constant and mechanical properties of a layer in a semiconductor device is at least partly a function of e-beam or UV beam, cure or dosage.
Consequently, for best electrical performance, a dosage that minimizes the dielectric constant is required, whereas for best mechanical strength a dosage that maximizes material properties such as cohesive strength is desired. The dosage values these two different requirements are usually not the same. Hence there is an advantage in being able to do region specific exposures for a given ILD layer with the goal of maximizing electrical performance in one region while also maximizing mechanical strength or certain chemical property in another region.